The present invention relates to flow stations for measuring the velocity of air moving through a duct and, in particular, to a static pressure sensor for use with such a flow station to measure the static pressure within a duct and which is not sensitive to the direction of air moving across the sensor.
Flow stations are devices that are inserted into ventilation air ducts within a building to measure the actual flow of air therethrough to some reasonable degree of accuracy. The impact of energy conservation and variable air volume systems has resulted in an increase in the use of these devices since they can be used to reduce energy consumption by insuring that only a sufficient amount of air is supplied to a zone to meet desired conditions.
The measurement of air flow in a duct system has always been difficult and somewhat unreliable. A number of devices have been available in the past and include velometers, anemometers, hot wire anemometers, pitot tubes, orifice plates, venturis, elbow meters, and flow nozzles. Some of these are localized velocity measuring devices and require multiple readings and an overall average to obtain an average duct velocity or flow. Others read an average flow rate but require individual calibration and require a severe pressure drop which represents lost energy and, therefore, inefficiency.
To solve these problems, the prior art introduced a multi-pitot tube self-averaging flow station. The rationale for this approach is that the pitot tube when properly designed is an inherently accurate sensor for measuring velocity pressure and does not require calibration. By introducing multi-pitot tubes located at equal duct area points and averaging the output of the pitot tubes, an accurate measure of average duct velocity and consequently average duct flow is obtained. The pressure drop in such a flow station is considerably less than a conventional orifice plate. The velocity pressures measured at the multiple points by the flow station is averaged by connecting each total pressure pick up into a first common manifold and each static pressure pick up into a second common manifold. The two manifolds then represent the average of the total and of the static pressures. Subtraction of these two pressures yields the average velocity pressure since the total pressure picked up by a pitot tube is a composite of velocity pressure and static pressure. Then, finally, in order to obtain velocity, or the amount of air moving through the duct, it is necessary to take the square root of the velocity pressure.
It can be shown that the velocity pressure, and as a result the total pressure, can vary substantially across the duct and, therefore, a plurality of total pressure pick up tubes is required for the average. However, the prior art has recognized that static pressure does not vary drastically across the duct and, therefore, only one static pressure pick up tube is required for an accurate measurement of the static pressure within the duct. Although more than one static pressure sensor may be required for larger ducts, as a general rule only one static pressure sensor is needed for a substantially accurate measurement of static pressure.
The prior art also found that air flowing through the duct is turbulent and, therefore, has a rotational component to it. It was found that both the total pressure pick up tubes and the static pressure pick up tubes were direction sensitive depending upon the direction of the air as it moved past the opening of the pick up tube. To eliminate this direction dependency, the prior art relied upon either a honeycomb of tubes for straightening the air or flow columnators at each pick up point. However, each approach adds material which restricts the size of the air duct thus creating undesired pressure drops. The purpose of the present invention is to eliminate this extra material in the duct thus reducing the pressure drop while yet retaining the direction insensitivity of the flow station. This objective can be accomplished by providing a static pressure pick up tube which is aligned so as to be insensitive to the direction of air moving passed it. In a more refined version of the invention, the static pressure sensor is self-aligning with air movement.